The present invention relates to a ROM memory cell not decodable by visual inspection.
As is known, ROM memory cells not decodable by visual inspection are used inside smart devices, such as smart cards, to store confidential data (for example personal identification codes to be used in bank transactions or with SIM cards).
ROM memory cells not decodable by visual inspection are also used to store suitable algorithms which can encrypt confidential data.
In both cases, to make the data contained in the ROM memory cells difficult to interpret externally, it is necessary to provide the cells with a protective screen, made from a dedicated metallization layer covering all of the ROM memory cells and not having any other function.
In general, in CMOS integration processes, the ROM memory cells are represented by NMOS transistors having two different values of the threshold voltage Vth (for example Vth=0.7 V and Vth=5.0 V).
Reading of the ROM memory cell content is carried out by feeding the NMOS transistors with a drain-source voltage VDS of approximately 1.0 V and a gatesource voltage VGS comprised between the two aforementioned threshold voltage values (for example the gate-source voltage VGS can have a value of approximately 2.5 V). The obtained drain-source current IDS is then compared with a reference current IREF which, for example, can have a value corresponding to the half-sum of the drain-source currents of an NMOS transistor with a high threshold (Vth=5.0 V), and of a NMOS transistor with a low threshold (Vth=0.7 V).
With reference to FIG. 1 showing the characteristic which associates the gate-source voltage VGS with the drain-source current IDS of an NMOS transistor, if the drain-source current IDS is greater than the reference current IREF (curve xe2x80x9cRxe2x80x9d), the NMOS transistor is at a logic state xe2x80x9c1xe2x80x9d, corresponding to the switched on condition. On the other hand, if the drain-source current IDS is lower than the reference current IREF, the NMOS transistor is at a logic state xe2x80x9c0xe2x80x9d corresponding to the switched off condition.
It is known that the ROM memory cells incorporated in the smart devices cannot be of the flash EEPROM or EPROM type since both can have problems of retention of the charge stored, which, over a period of time, can lead to loss of the data stored. It is therefore necessary to use alternative approaches. For this purpose, the NMOS transistor representing the logic state xe2x80x9c1xe2x80x9d (low-threshold transistor) is generally carried out such that it is altogether similar to standard NMOS transistors present in the smart device, whereas the NMOSFET transistor representing the logic state xe2x80x9c0xe2x80x9d (high-threshold transistor) is carried out using two alternative approaches.
For this purpose, FIG. 2 shows a transverse cross-section of a first embodiment of a ROM memory cell 1 incorporated in a smart device 50 and comprising a semiconductor material substrate 2 with Pxe2x88x92-type conductivity.
The ROM memory cell 1 includes a first low-threshold transistor 4 and a second high-threshold transistor 7. The first transistor 4 is formed by a first conductive region 3, of type N+, defining a drain region, and by a first source region 6a, of type N+, both formed in a first portion 5 of the substrate 2. The second transistor 7 is formed by a second source region 6b, of type N+, providing in a second portion 8 of the substrate 2 adjacent to the first portion 5 and joined to the first source region 6a such as to form a second conductive region 6, and by a third conductive region 9, of type N+, defining a drain region of the second transistor 7.
The ROM memory cell 1 additionally comprises first and second extension regions 10, 11, of type N, operating respectively as a drain extension region and a source extension region of the first transistor 4, and third and fourth extension regions 12, 13 of type N operating respectively as a source extension region and a drain extension region of the second transistor 7.
The first and second extension regions 10, 11 extend laterally and in a position adjacent respectively to the first and second conductive regions 3, 6, and face one another. The first and second extension regions 10, 11, delimit a portion of substrate 2 forming a first channel region 15. Similarly, the third and fourth extension regions 12, 13 extend laterally and in a position adjacent respectively to the second and third conductive regions 6, 9, and face one another. The third and fourth extension regions 12, 13 delimit a portion of substrate 2 forming a second channel region 18.
The second channel region 18 accommodates an implanted region 19, which is more doped than the substrate 2, to increase the threshold voltage of the second transistor 7.
Above the first channel region 15, there is formed a first gate region 20 of polycrystalline silicon of the first transistor 4. Similarly, above the second channel region 18, there is formed a second gate region 21 of polycrystalline silicon of the second transistor 7. The first gate region 20 is isolated from the first channel region 15 by means of a first gate oxide layer 24, whereas the second gate region 21 is isolated from the second channel region 18 by means of a second gate oxide layer 25. Above the first and second gate regions 20, 21, and above the first, second and third conductive regions 3, 6, 9, there are present a first gate contact region 31, a second gate contact region 32, a first drain contact region 33, a common source contact region 34, and a second drain contact region 35 of a metal silicide, for example titanium. The first gate region 20 and the first gate contact region 31 are laterally adjoined by first oxide spacers 40. Similarly, the second gate region 21 and the second gate contact region 32 are laterally adjoined by second oxide spacers 41.
This first embodiment of the ROM memory cell 1 has the disadvantage that it is relatively costly to carry out, since it is necessary to add to the process phases commonly used additional phases of photolithography and implantation for defining the implanted region 19.
FIG. 3 shows a cross-section of a second embodiment of the ROM memory cell 1, in which parts corresponding to the first embodiment in FIG. 2 have been provided with the same reference numbers. In the second embodiment, the formation of the implanted region 19 and of the third and fourth extension regions 12, 13 is not provided.
In particular, the lack of definition of the third and fourth extension regions 12, 13 means that the second gate region 21 is not fully superimposed on the second channel region 18, which also extends partially below the second oxide spacers 41. In these design conditions, the application of a gate-source voltage VGS to the second transistor 7 is not sufficient to take the latter into a condition of conduction, i.e., to form a continuous reversal region between the second source region 6b and its drain region 9. This is owing to the fact that the portions of the second channel region 18 which are below the second spacers 41 cannot reverse their conductivity (from N to P), in the short period of time in which ROM memory cell 1 reading takes place.
Although this second embodiment of the ROM memory cell 1 has reduced manufacturing costs, since it does not require the operations of photolithography and implantation dedicated to the formation of the implanted region 19, it nevertheless has the disadvantage that it is difficult to control, since its satisfactory functioning depends on the dimensions of the oxide spacers 41.
The technical problem of the present invention is to provide a ROM memory cell not decodable by visual inspection.
According to one embodiment of the invention, the ROM memory cell comprises a substrate of semiconductor material having a first conductivity type, in particular Pxe2x88x92. A first MOS device having a first threshold voltage is formed in a first portion of the substrate, and a MOS device having a second threshold voltage, greater than the first threshold voltage, is formed in a second portion of the substrate adjacent to the first portion. The second MOS device is a diode, reverse biased during a reading phase of the ROM memory cell and includes an anode region having the first conductivity type and a cathode region having a second conductivity type. The anode region has a level of doping higher than that of the substrate.